A metal-air battery is a battery using a metal such as lithium (Li), zinc (Zn), aluminum (Al), magnesium (Mg), iron (Fe), calcium (Ca) and sodium (Na) in a metal electrode (negative electrode) and using oxygen in the air as a positive electrode active material. In addition, a metal-air battery produces electricity by reacting metal ions of a negative electrode with oxygen, and unlike existing secondary batteries, does not necessarily have a positive electrode active material inside the battery in advance, which makes weight lightening possible. In addition, a negative electrode material may be stored in large quantities in the container, and theoretically high capacity and high energy density may be obtained.
A metal-air battery is formed with a metal electrode (negative electrode) and an oxygen air electrode (positive electrode). When discharged, metal ions are formed due to oxidation of the metal electrode, and the produced metal ions migrate to the oxygen air electrode through an electrolyte. In the oxygen air electrode, external oxygen is dissolved in the electrolyte inside the pores of the oxygen positive electrode and is reduced.
Particularly among metal-air batteries, a lithium-air battery is generally provided with a negative electrode capable of intercalating/deintercalating lithium ions, and a positive electrode including a redox catalyst of oxygen with oxygen in the air as a positive electrode active material, and is provided with a lithium ion conductive medium between the positive electrode and the negative electrode. Theoretical energy density of a lithium-air battery is 3000 Wh/kg or greater, and this corresponds to energy density of approximately 10 times compared to lithium ion batteries. Moreover, a lithium-air battery is environmental-friendly, and is capable of providing more improved safety compared to lithium ion batteries, and therefore, much developments have been progressed.
Important factors determining electrochemical properties of a lithium-air battery include an electrolyte system, a positive electrode structure, an excellent air reduction electrode catalyst, types of a carbon support, an oxygen pressure and the like, and a formula of the reaction occurring in a lithium-air secondary battery is as shown in the following Reaction Formula 1.Oxidation electrode: Li(s)↔Li++e−Reduction electrode: 4Li+O2→2Li2OV=2.91 V2Li+O2→Li2O2V=3.10 V  [Reaction Formula 1]
In other words, lithium produced from a negative electrode during discharge meets oxygen gas of a positive electrode to produce lithium oxide, and the oxygen is reduced (oxygen reduction reaction: ORR) to generate oxygen anions. On the contrary, lithium oxide is reduced during charge, and oxygen gas is produced while oxygen is oxidized (oxygen evolution reaction: OER).
The solid lithium oxide produced during discharge is not favorably dissolved in an organic solvent and is present as a solid oxide. It is accumulated in a reaction site of a carbon electrode, a positive electrode, blocking oxygen channels and inhibiting diffusion of oxygen. In other words, the solid lithium oxide blocks pores of carbon, a positive electrode, as well as inhibiting contact between oxygen and lithium ions making lithium oxide formation difficult, and therefore, capacity is difficult to be revealed and secondary battery properties decline. In addition, charge transfer is inhibited during charge due to a side reaction deposit forming high resistance and overvoltage, and this leads to a liquid electrolyte decomposition reaction causing a problem of battery degradation.